A significant challenge for magnetic resonance (MR) imaging at fields above 3 Tesla can be the relative lack of available radio frequency (RF) coils. In addition, the design of RF coils for high field can be complicated since, at the higher RF frequencies, there may be a more complex interaction of the RF excitation field with the sample. This can lead to an excitation inhomogeneity due to wavelength effects and complex twisted B1+ and B1− profiles which differ significantly from those seen at low field. Predicting the B1+ and B1− profiles which differ significantly from those seen at low field. Predicting the behavior of RF coil designs for high field MR can, at times, require a full wave simulation of the electromagnetic fields, since low-field quasi-static approximations no longer apply.
One simple RF coil design which can be employed for high field imaging is the transmit-receive surface coil. Even at low fields, this coil design suffers from transmit and receive profiles which are steeply reduced with depth into the sample. However, given the complexities of creating a uniform excitation in anything but the smallest volumes at high field, the transmit receive surface coil can provide an attractive solution for imaging small regions of interest in the body and for acquiring preliminary data as improved application-specific coils designs are developed. With increasing field, however, the efficiency of the transmit-receive surface coil can diminish due to the B1+ and B1− profiles being twisted in opposite directions.
Improvements in SNR can be obtained through the use of receive arrays optimized to particular regions of interest in the body. At 3 Tesla, improvements in carotid imaging can achieved by using a dedicated 8-channel carotid receive array. (See Hinton-Yates et al., Top Magn. Reson. Imaging 2007, 18:5; 389-400). At higher fields, transmit body coils may not be readily available, and uniform excitation in the body can be difficult to achieve. Therefore, any 7 Tesla coil should include a transmit capability, which can likely increase the complexity of the design thereof. In addition, the complex twisted B1+ and B1− profiles in human tissue at high RF frequencies can likely require a full wave electromagnetic simulation to accurately predict the performance of any given coil design, which provides additional complexity.
Thus, it may be beneficial to address and/or overcome at least some of the deficiencies described herein above.